The automobile industry has increasing requirements on the quality of glazings that are supplied to it. In particular, in addition to optical quality and dimensional precision, it requires a very great precision in the curves of the glazings and demands new shapes.
In the bending processes used so far (for example, of the type described in European Pat. No. 3,391), a planar glass plate, arriving on a conveyor after having been brought to a high temperature, is taken by a partial vacuum grasping device acting on the upper face of the glass plate. The glass plate is lifted to allow a bending frame with a curved profile, open in its center and with an upturned concavity, to be brought under it. Then the glass plate is placed or allowed to drop on the bending form, on which it sags under its own weight to take the shape of the bending form.
The profile of the glass plate thus bent then corresponds rather well, on its edges, to the desired profile because the plates sags to rest by its edges on the bending form, but the curve of the plate in its center is hard to control. If the sagging time is too long, and/or if the temperature of the glass is a little too high, an excessive sagging occurs in the center of the glass plate, which results in a profile with too pronounced a curvature in the center. Similarly, if the temperature is too low and/or the sagging time is too short, too little sagging occurs in the center of the glass plate, which results in a profile with too little curvature in the center.
Further, the bending frame can lightly mark the glass and be the cause of a slight flattening on the edge of the glass plate.
Further, some curves are not attainable by processes of this type--in particular, those that are too pronounced near the edge of the glass plate and those that have to assume narrow and/or angular portions of the glass plates.
Further also, the automobile industry with increasing frequency requires glazings carrying enamel deposits, for example, as narrow peripheral strips, particularly to mask the glue which will be used to fasten the glazings and window openings of automobile bodies, as networks covering the entire surface to constitute sunscreens, or as strips or various patterns to serve as decorative elements. These enamel deposits add an additional difficulty to the production of glazings, to their handling, and to their working in general because of the fragility of the enamel and its still liquid or pasty consistency at the beginning of the bending operation.
In the above mentioned bending process, the face of the glass plate which carries the enamel is directed upwardly to avoid damage on the conveyor rollers, and therefore it comes in contact with a grasping device. Accordingly, precautions must be taken to avoid marking the glass plates with enamel coming from previously treated glass plates, which enamel will have fouled the grasping device.